MENTAL EVOLUTION IN THE PRIMATES 127 



prospective food and permits the animal to see it placed 

 beyond a particular one of the doorways. Thereupon, by 

 use of a suitable screen the animal's view of the food is cut 

 off, and the three doorways or reaction areas therefore 

 appear to it exactly ahke. For a definite period, say ten 

 minutes, the animal awaits opportunity to respond. It then 

 is released and makes choice among the doorways. As thus 

 far indicated by observations, this demand for response on 

 the basis of prior sensory experience is more easily and 

 successfully met by the monkey than by the rat; by the ape 

 than by the monkey, and by man than by ape. The temporal 

 span of memory, or period during which the animal may be 

 kept waiting without losing its power of correct response, 

 increases very rapidly from rat to man. No experiments with 

 lemurs are available, so we refer to the rat instead. Although 

 further observation may essentially modify the findings, it 

 is indicated at present that the rat ceases to respond adap- 

 tively after delays exceeding a few seconds, whereas 

 monkeys, apes and men may succeed after many hours. 



An essentially different and more exacting test of the 

 existence of memory processes has recently been employed 

 to exhibit the ability of various primates. The subject 

 faces a group of boxes which are visually alike except, 

 for instance, in color. Into one of the boxes it sees food put. 

 A screen is then drawn between the subject and the boxes, 

 and the latter are interchanged so that the food container 

 cannot be located by its initial position but must instead 

 be identified by the visual quality of color. After a stated 

 interval of delay, which in the course of experimentation has 

 ranged from a few seconds to nearly an hour, the screen is 

 removed and the animal given opportunity to seek the food. 

 At the present writing, in addition to man, only the chim- 

 panzee and gorilla have demonstrated their ability to react 

 successfully in this type of situation after delays of at least 

 ten minutes. It may not safely be inferred that all other 

 types of organism are incapable of such response, but it is 

 safe to infer tentatively that success in this type of memory 

 experiment is more readily and more frequently achieved 

 as we progress from the various orders of mammal, through 

 the several classes of primate to man. 



